1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV). More particularly, it pertains to the control of transitions between electric-drive and parallel-drive operation of the powertrain.
2. Description of the Prior Art
One of the more promising technologies to improve vehicle fuel efficiency is to hybridize a conventional vehicle powertrain with an electric drive system that consists of one or more electric machine and a high-voltage battery. The automotive industry is investing resources in developing such hybridized powertrain concepts and configurations in order to accelerate vehicle electrification and ultimately move towards plug-in hybrid or battery electric vehicles. These hybrid powertrain configurations can be categorized into three types, i.e., series hybrid system, parallel hybrid system, and complex hybrid systems. Complex hybrid systems provide the characteristics of both a series and parallel configurations. One such complex hybrid configuration is a Dual-Drive hybrid powertrain.
Hybrid electric vehicle (HEV) powertrains are important to developing environmentally friendly and fuel efficient vehicles. A “Dual-Drive” full hybrid electric vehicle overcomes some deficiencies over existing hybrid powertrain architectures due to their kinematic arrangement of engine, motors and driveline components. This hybrid powertrain comprised of conventional powertrain components as its base with an electric motor on the rear axle, and a crank integrated starter generator, engine and transmission on the front axle forms a complex configuration which provides fuel economy improvement over a conventional powertrain. However, due to this complex configuration, transitioning between electric and parallel hybrid drive modes presents a unique powertrain control challenge and requires coordinated vehicle and subsystem controls.
Since the Dual-Drive system provides independent propulsion capability on each drive axle, a need exists to control torque delivery between the front and rear axles during transitions between electric-drive and parallel-drive modes while providing responsive vehicle performance and acceptable drivability. Furthermore, due to the multiple degrees of freedom in controlling the powertrain under various operating modes, a need exists to develop an energy management control system to perform selection of the powertrain operating mode and blending of torque, speed, and power from multiple power sources such that the benefit from this hybridization is maximized. In addition, since the Dual-Drive hybrid powertrain uses a fixed stepped-ratio automatic transmission, a need exists for coordination of transmission control (i.e. shifts, engagements/disengagement, etc) during engine start/stops, regenerative braking and powertrain operating mode transitions.